Generally, prior art magnetic recording systems employ AC bias or saturated recording techniques. AC bias recording is used to improve the linearity of the recording channel, but reduces the high frequency response of the recording channel. Saturation recording can only record two magnetization levels and produces higher noise. In both methods, data signals are written in the baseband, i.e., the write signal contains a very high signal energy in the baseband. The baseband of a recording channel is defined as the range of frequencies or frequency band in which a sinusoidal write current will produce a non-zero readback signal when using DC or AC bias, or saturating or non-saturating recording.
In those prior art systems where stored information is being read out while new data is being written, such as in disk drives that have prerecorded servo signals in a buried layer of the magnetic disk that are being read out while write data signals are being registered on a separate layer of the disk, the frequencies of both the read servo signal and the newly recorded data signal are within the baseband frequency range. It is necessary, therefore, to separate the two signals by the use of critical filters on the write side and/or on the read side of the recording channel. Filtering causes an undesirable attenuation and phase distortion of the signals and other deleterious effects, besides adding to the cost, mass and volume of the processing system. Also, when AC bias is used for writing data, the phases of the respective bias and data signals result in a varying peak shift depending on the actual phase between the data pulses and bias signal zero crossings.
In prior art systems, the frequency of the write data signal is always in the baseband, as exemplified by FIG. 1A, representing a saturated recording method. An auxiliary input signal having a frequency above the baseband, such as an AC bias signal, may be used but not recorded in order to linearize the channel. The bias signal is a high frequency sinewave that is added to the baseband signal. This high frequency write signal cannot be stored in the magnetic medium because the thin magnetic recording media on the disk cannot sustain the short wavelength associated with this frequency; it is outside the baseband, as illustrated in FIG. 1B.
It would be desirable for instance for a head positioning servo system in a disk file to provide a magnetic recording system which operates without AC bias or saturated recording, and which obviates the need for complex and costly filters to separate the data signal from low frequency servo information.